How to change dimension of circles without scaling it?
One way of doing this (it may not fit your workflow, though) is using an array modifier, fitting and following a circled curve (you can create a mesh circle, then convert it to curve with ALTC in object mode):

First, you create a thin rectangle that will fit and then follow a curve, with modifiers. A thinner rectangle will give (as replied by array) higher resolution of the curved shape.



which produces this:



to make another circle, of the same width, duplicate both curve and rectangle, first:



then, scaling the curve, will keep the same width:



another bigger curve:



If you need to have just static meshes, you can then apply (in order) modifers... (maybe keep a "dynamic" copy to create new ones...)